Why do we startle when we fall asleep?

You're peacefully drifting off to sleep when suddenly you feel as if you're falling into a void and you startle awake. This rather unpleasant sensation is known as a ‘hypnic jerk’ and is a common occurrence. It is an involuntary muscle spasm, a bit like a hiccup, also known as ‘myoclonus’. Sometimes, this sensation is accompanied by the feeling or the dream ... Show More